Impact of non-linear distortions on different modulation schemes in analog CATV distribution systems

Over the last years several solutions, based on either a directly modulated DFB laser or an external modulator, have been used in AM CATV systems. The individual effects of clipping and chirping distortions on the performances of CATV systems using directly modulated lasers have been analysed in different papers in terms of CTB, CSO and NLD. The aim of this paper is to analyse and compare the different modulation schemes taking simultaneously into account all the effects influencing the transmission performance. Here we consider four different cases. The first refers to directly modulated DFB lasers (DM-DFB), the other three relate to external modulation using a Mach-Zehnder modulator (MZ). In particular these cases refer to conventional MZ modulators (C-MZ), to linearized modulators (L-MZ) and to a predistorted modulation scheme (P-MZ). Nonlinear distortion limits the performance of CATV distribution systems. In particular, besides clipping, the direct modulation of lasers is significantly affected by chirping induced distortions. A reduction of the chirping/dispersion effect, obtained either by using dispersion shifted fiber, or reducing the overall lengths, can rend the direct modulation of the laser still applicable. In any case the predistorted technique appears to be the best solution concerning the transmission performance.